Method of exposing printed wiring boards having through holes

ABSTRACT

A method of exposing the resist on a printed wiring board having through holes for connecting planar circuit patterns on the plane surfaces of the substrate is disclosed, wherein an electron beam direct writing system is utilized for scanning a converging electron beam as the exposure energy. The electron beam produced by the system enters the opening of the through hole, forming an inclination angle different from 0 degrees and up to 7 degrees with respect to the normal to the opening area of the through hole, such that the resist on the wall surface of the through hole is exposed simultaneously both by the direct and the reflected electron beam. The amount of exposure per unit area of the opening of the through hole may be controlled to 2 h/r times the amount of exposure per unit area over the planar surface of the substrate, wherein h and r are the depth and the radius of the through holes, respectively. A reflector plate may be disposed at the bottom opening of the through holes to further enhance the utilization efficiency of the electron beam during through-hole wall surface exposure.

This application is a continuation of application Ser. No. 07/544,646,filed Jun. 27, 1992 now U.S. Pat. No. 5,147,760.

BACKGROUND OF THE INVENTION

This invention relates to a method of exposing printed wiring boards,such as multilayer printed wiring boards, which comprise, in addition tocircuit patterns formed on plane surfaces, circuit patterns extendingthrough through-holes so as to connect the planar circuit patterns witheach other.

In the case of multilayer printed circuit or wiring boards, the circuitpatterns formed on planar surfaces of the boards are connected viacircuit patterns extending through through-holes formed in the boards.Such circuit patterns, which include patterns extending throughthrough-holes as well as patterns formed on planar surfaces of theboards, may be formed by means of the photolithography method utilizingmasks.

FIG. 1 shows a vertical section of a device for exposing printed wiringboards having through-holes, which device is disclosed in a recentarticle by A. Hoshino et al.: Mitsubishi Denki Giho [Technical Reportsof Mitsubishi Electric Corporation], vol. 63. No. 2, 1989, pp. 75through 78. In the figure, substrate 1 of a printed wiring board havinga plurality of through holes 2 is covered by a negative type photoresist3 formed on the substrate 1 by the electro-deposition method. It isnoted that the resist 3 covers the wall surfaces of the through holes 2as well as the plane surfaces of the substrate 1. Mask films 4, havingpatterns corresponding to the circuit patterns, are disposed on bothmain planar surfaces of the substrate 1. Each one of the two units ofthe exposing device, disposed above and below the substrate 1, comprisesa lamp 5 for exposure, a reflector mirror 6, and a slit 7, and emitsultraviolet light 8 diverging by an angle θ with respect to the centraloptical axis of the lamp 5 normal to the main plane surfaces of thesubstrate 1. The exposing device, or the substrate 1, is translated inthe direction A during the exposure.

The resist pattern corresponding to the desired circuit pattern isformed on the substrate 1 as follows. First, all the surfaces of thesubstrate 1 having through holes 2 are covered by resist 3 by means, forexample, of the electrodeposition method. Then, the mask films 4 aredisposed on the plane surfaces of the substrate 1, so that the resist 3is exposed to the ultraviolet light 8 which is selectively transmittedthrough the mask films 4. Thereafter, the substrate 1 is exposed bymeans of the exposing device as shown in FIG. 1, wherein the exposingdevice or the substrate 1 is translated in the direction A, while thedivergence angle θ of the ultraviolet light 8 is defined by the slit 7.Thus, the portions of the resist 3 on the plane surfaces of thesubstrate 1 corresponding to the circuit patterns that are to be formedthereon are selectively exposed; in addition, the whole wall surfaceareas of the resist 3 within the through holes 2 of the substrate 1 areexposed. After the above sequence of operations, the resist 3 on thesubstrate 1 is developed; thus, the portions of the negative type resist3 which have become insoluble as a result of the exposure to theultraviolet light are retained and a resist pattern corresponding to thecircuit pattern that is to be formed on the printed wiring board isformed on the substrate 1.

However, the above method of exposing printed wiring boards havingthrough holes has a disadvantage. Namely, the amount of exposure of thewall surfaces of the through holes 2 is small compared with the amountof exposure of the planar surfaces of the substrate 1. When thethickness of the substrate 1 is 1.6 mm and the diameter of the throughholes 2 is 0.4 mm, the ratio, Wt/Ws, of the exposure, Wt, of the wallsurfaces of the through holes 2, to the exposure, Ws, of the planesurfaces of the substrate 1, varies as shown in FIG. 2 as a function ofthe divergence angle θ of the ultraviolet light 8; in FIG. 2, theexposure ratio Wt/Ws is plotted in percent (%) along the ordinate whilethe irradiation divergence angle θ of the ultraviolet light 8 is plottedalong the abscissa. As shown in FIG. 2, in the case where the angle θ iswithin a commonly employed range of about 30 to 40 degrees, the exposureratio Wt/Ws is about 20%. Thus, the amount of exposure Wt of the wallsurfaces of the through holes 2 is small compared with the amount ofexposure Ws of the plane surfaces of the substrate 1. It is impossible,however, to selectively increase the level of exposure of the wallsurfaces of the through holes 2 while retaining the level of exposure ofthe planar surfaces of the substrate 1 to an appropriate level. Thus, inorder to obtain sufficient exposure on the wall surfaces of the throughholes 2, it is necessary to increase the whole exposure level and to setthis exposure level in accordance with the condition for obtaining asufficient exposure level of the wall surfaces of the through holes 2 ofthe substrate 1. This, however, results in an undesirable increase ofthe resist pattern width on the planar surfaces of the substrate 1; thisfact is clearly shown in FIG. 3, where the relations of the circuitpattern width (plotted in μm along the ordinate) with respect to thelevel of exposure (plotted along the abscissa in mJ/cm²) are shown inthe three cases where the designed values of the pattern width are 100μm, 150 μm, and 200 μm, respectively; when the exposure level is in therange (shaded in the figure) in which the exposure level of the throughholes is proper, the width of the planar patterns on the substrate 1 isincreased by about 20% due to excessive exposure. Thus, the above methodof exposure has the disadvantage that high density printed circuitboards with finer circuit patterns cannot be produced with accuracy andreliability.

SUMMARY OF THE INVENTION

It is therefore a primary object of this invention to provide a methodof exposing a resist on a substrate of a printed wiring board havingthrough holes, according to which the wall surfaces of the through holescan be exposed stably and reliably without increasing the pattern widthof the circuit pattern to be formed on the planar surfaces of thesubstrate.

The above object is accomplished according to the principle of thisinvention by a method of exposing a resist formed on a substrate havingat least one cylindrical through hole, wherein an opening of the throughhole and the planar surface of the substrate are scanned by an exposureenergy beam (such as a converging electron beam generated by an electronbeam direct writing system) according to a predetermined pattern, and,during irradiation over the opening area of the through hole, anirradiation inclination angle is formed with respect to a normal to theopening area of the through hole, such that an incident-beam irradiatedportion on the wall surface, whose resist is directly irradiated withthe exposure energy beam, and a reflected-beam irradiated portion on thewall surface, whose resist is irradiated with a reflected exposureenergy beam generated by a reflection of the incident exposure energybeam at the resist on the incident-beam irradiated portion, are exposedsimultaneously. The irradiation inclination angle is preferred to becontrolled to a value from 0 degrees up to 7 degrees; further, theinclination angle may be controlled in accordance with the depth andradius of the through hole.

Preferably, especially in the case where the depth of the through holeis large, an amount of exposure per unit area over the opening area ofthe through hole is controlled to not be less than 2 h/r times theamount of exposure per unit area over the planar surface of thesubstrate, wherein h designates the depth of the through hole and rdesignates the radius thereof. By this measure, a sufficient exposurelevel of the well surface of the through hole is ensured.

According to another aspect of this invention, which may be combinedwith the first aspect as defined above, the above object is accomplishedby a method of exposing a resist formed on a substrate of a printedwiring board having at least one through hole, wherein an opening of thethrough hole and the planar surface of the substrate are irradiated witha converging exposure energy beam (such as a converging electron beamgenerated by an electron beam direct writing system) according to apredetermined pattern, and, during irradiation of the through hole, theconverging exposure energy beam entereing into the through hole via anopening thereof is reflected by means of a reflector plate disposed atan opposite opening of the through hole, such that the resist on thewall surface of the through hole is exposed simultaneously both by theconverging exposure energy beam directly incident thereon and by theexposure energy beam reflected at the reflector plate.

The novel features which are believed to be characteristic of thisinvention are set forth with particularly in the appended claims. Thisinvention itself, however, may best be understood, together with furtherobjects and advantages thereof, by reference to the detailed descriptionof the preferred embodiments taken in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a conventional exposure deviceduring the exposure operation of a printed wiring board having throughholes;

FIG. 2 is a diagram showing the exposure ratio curve which representsthe exposure level of the through hole with respect to the exposure ofthe planar surface of the substrate in the case of the exposureoperation effected by means of the device of FIG. 1;

FIG. 3 is a diagram showing the increase of the pattern width over thedesigned values thereof, which increase takes place in the case of theexposure method utilizing the device of FIG. 1;

FIG. 4 is a diagrammatic elevational sectional view of the electron beamdirect writing system utilized by an exposure method according to thisinvention;

FIG. 5 is a perspective view of the main portion of the system of FIG.4;

FIG. 6 is a view similar to that of FIG. 4, but showing a systemutilized by an exposure method according to a second aspect of thisinvention; and

FIG. 7 shows the relation between the atomic number and theback-scattered electron generation ratio.

In the drawings, the same reference numerals represent the same orcorresponding parts or portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 4 to 7 of the drawings, embodiments according tothis invention will be described.

Let us first describe a first embodiment of this invention referring toFIGS. 4 and 5, which show an overall sectional view and an enlargedperspective view, respectively, of an electron beam direct writingsystem utilizing a convergent electron beam as the exposure energy beamfor exposing printed wiring boards. In the figures, the electron beamdirect writing system includes: a pattern generator 11, D/A converter 12for deflection of the electron beam, deflection power source 13,deflection coils 14, D/A converter 15 for controlling electron beamcurrent, electron beam current source 16, cathode 17 of the electrongenerating triode disposed in the electron beam column C, focusing powersource 18, and focusing coil 19. The converging electron beam 20produced in the electron beam column C irradiates and exposes thesubstrate 1 of a printed wiring board having through holes 2 (only oneis shown in the figure) and covered with resist 3. The substate 1 isdisposed in a vacuum working chamber W coupled to the base of theelectron beam column C, and, during exposure of the cylindrical throughhole 2, the converging electron beam 20 produced in the column C entersinto the cylindrical through hole 2 through the opening area 21 of thethrough hole 2, forming an inclination angle δ with respect to thenormal N to the opening area 21 of the through hole 2, as best shown inFIG. 5, where reference character r designates the radius of the throughhole 2 and reference character h designates the depth of the throughhole 2. The converging electron beam 20 is reflected at theincident-beam irradiated portion 22 on the wall surface of the throughhole 2, to fall on the reflected-beam irradiated portion 23 as areflected beam. The resist 3, which may be made of a typical photoresistgenerally utilized in the photolithography process, is formed on thecopper-clad substrate 1 by the electrodeposition method; the resist 3covers the whole wall surface of the through hole 2 of the substrate 1,as well as the planar surfaces of the substrate 1.

The exposure operation by the electron beam direct writing system ofFIG. 4 is effected as follows. The CAD (computer aided design) data forthe circuit pattern to be formed on the printed wiring board 1 isconverted into the direct writing vector data in a CAM (computer aidedmanufacturing) work-station (not shown); on receiving this data, thepattern generator 11 generates scanning signals corresponding to thecircuit pattern to be formed on the printed wiring board 1. Thus, anelectron beam is emitted from the cathode 17 of the triode in responseto the current supplied thereto from the electron beam current source 16which is controlled by the output of the D/A converter 15 for theelectron beam current, which D/A converter converts a digital signalreceived from the pattern generator 11 into a corresponding analogsignal. Further, the electron beam emitted from the cathode 17 isconverged and focused by the magnetic field generated by the focusingcoil 19, which is supplied with a focusing current from the focusingpower source 18, and which is also controlled by an output of thepattern generator 11. The converging (i.e., focused) electron beam 20thus formed is deflected by the deflection coils 14 in response toscanning signals generated by the pattern generator 11. Namely, thedigital scanning signals generated by the pattern generator 11concerning the scanning position and the scanning speed of the electronbeam 20 are converted at a high speed into corresponding analog signalsby means of the deflection D/A converter 12; in accordance with theanalog signals received from the D/A converter 12, the deflection powersource 13 supplies necessary currents to the deflection coils 14 suchthat the scanning position and the scanning speed are controlled by themagnetic field produced by the deflection coils 14, in accordance withthe scanning signal generated by the pattern generator 11. Thus, theconverging electron beam 20 is scanned on the substrate 1 according tothe predetermined circuit pattern.

During the above-described exposure process, the converging electronbeam 20 is irradiated over the whole region of the opening 21 of thethrough hole 2 in such a manner that the converging electron beam 20forms a predetermined inclination angle δ different from 0 degrees withrespect to the normal N to the opening area 21 of the through hole 2(see FIG. 5). The inclination angle δ of the converging electron beam 20with respect to the normal N to the opening area 21 of the through hole2 is preferred to be controlled to a value different from 0 degrees upto 7 degrees. Further, the inclination angle is preferred to bedetermined on the basis of the radius r and the depth h of through hole2; more specifically it may be determined on the basis of the ratio r/hof the radius r and the depth h of the through hole 2. As a result ofthe formation of the inclination angle of the electron beam 20, theconverging electron beam 20 exposes not only the incident-beamirradiated portion 22 whose resist 3 covering the wall surface of thethrough hole 2 is directly irradiated by the converging electron beam20, but also the reflected-beam irradiated portion 23 simultaneously,whose resist 3 on the wall surface of the through hole 2 is irradated bythe reflected electron beam generated by the reflection of theconverging electron beam 20 at the resist 3 at the incident-beamirradiated portion 22. Thus, the utilization efficiency of theconverging electron beam 20 during the through-hole wall surfaceexposure is dramatically increased compared with the efficiency duringthe circuit pattern formation upon the upper plane surface of thesubstrate 1; hence the wall surfaces of the through holes 2 can beexposed without shadows formed thereon, and printed wiring boards havingthrough holes can be exposed with enhanced efficiency, reliability, andstability.

In the case of the electron beam direct writing system such as thatshown in FIG. 4, in contrast to the conventional ultraviolet light lampsutilized for conventional exposure processes, the amount of exposure perunit area (which may be measured in mJ/cm²) by the electron beam can becontrolled and varied from time to time precisely at a high variationspeed and with high accuracy; this control of the amount of exposure perunit area can be effected, for example, by (a) controlling the electronbeam current supplied to the cathode 17 of the electron beam generatingtriode in accordance with the command from the pattern generator 11 viathe D/A converter 15 and the electron beam current source 16, or (b)controlling the scanning speed of the converging electron beam 20 inaccordance with the command from the pattern generator 11, which commandis executed via the deflection D/A converter 12, the deflection powersource 13, and the deflection coils 14. Thus, according to a preferredaspect of this invention, the amount of exposure per unit area (measuredin mJ/cm²) over the opening area 21 of the through hole 2 is controlledto not less than 2 h/r (h being the depth and r the radius of thethrough hole 2, as shown in FIG. 5) times the amount of exposure perunit area (measured in mJ/cm²) over the plane surface of thesubstrate 1. It is noted in this connection that, as described above,the utilization efficiency of the electron beam during the exposure ofthe wall surface of the through holes 2 can be enhanced by irradiatingthe converging electron beam 20 at an inclination angle δ with respectto the normal N to the opening area 21 of the through hole 2; thus, inthe case where the depth h of the through hole 2 is relatively small,the above-mentioned control of the amount of exposure is not necessary.However, in the case where the depth h of the through hole 2 isrelatively great, the differential control of the exposure levels overthe plane surface of the substrate 1 and over the opening area of thethrough hole 2 is preferred; namely, since the area 2πr·h of the wallsurface of the cylindrical through hole 2 is equal to 2 h/r times thearea πr² of the opening area 21 of the through hole 2, the wall surfaceof the through hole 2 can be exposed with a more enhanced stability bycontrolling the amount of exposure per unit area over the opening area21 of the through hole 2 to not less than 2 h/r times the amount ofexposure per unit area during the planar pattern formation over theplane surface of the substrate 1. It is further noted in this connectionthat since the amount of exposure over the opening area 21 of thethrough hole 2 is controlled independently of the amount of exposureduring the planar pattern formation over the plane surface of thesubstrate 1, the circuit pattern width or the precision of the circuitpattern on the plane surface of the substrate 1 can be maintainedwithout degradation.

Referring now to FIG. 6 of the drawings, a method of exposure accordingto a second embodiment of this invention, which also utilizes anelectron beam direct writing system for producing a converging electronbeam as the exposure energy beam is described in FIG. 6, the electronbeam column C (comprising the cathode 17 of the triode, the focusingcoil 19, and the deflection coils 14), as well as the pattern generator11, the deflection D/A converter 12, deflection power source 13,electron beam current control D/A converter 15, electron beam currentsource 16, and the focusing power source 18, are organized and operatedsimilarly as in the case of the electron beam direct writing system ofFIG. 4. Further, the substrate 1 having through holes 2 and covered withresist 3 is disposed in the vacuum working chamber B, as in the case ofthe above-described system shown in FIG. 4; thus, for the descriptionthereof, reference should be made to the above-description of the firstembodiment of FIG. 4.

However, in the case of the system shown in FIG. 6, reflector plate 23is disposed within the working chamber W at the bottom opening of thethrough hole 2 opposite to the opening irradiated by the convergingelectron beam 20. Thus, as shown in FIG. 6, the reflected-beamirradiated portion 23, which is irradiated and exposed by a reflectedbeam of relatively low directivity produced by the reflection of aportion of the electron beam 20 at the incident-beam irradiated portion22 and at the reflector plate 23, is exposed simultaneously as theincident-beam irradiated portion 22, whose resist 3 covering the wallsurface of the through hole 2 is directly irradiated and exposed by theconverging electron beam 20. Thus, the utilization efficiency of theconverging electron beam 20 during the through-hole wall surfaceexposure is further increased and enhanced, compared with theutilization efficiency of the electron beam 20 during the circuitpattern formation upon the plane surfaces of the substrate 1; hence thewall surfaces of the through holes 2 can be exposed without shadowsformed thereon, and printed wiring boards 1 having through holes 2 canbe exposed with more enhanced efficiency, stability, and reliability.

FIG. 7 shows the relationship between the atomic number Z and theback-scattered electron generation ratio η, which relationship is takenfrom: "Electron/Ion Beam Handbook", p. 621, edited by the 132ndcommittee of Nihon Gakujutsu Shinkokai, and published by Nikkan KogyoShinbunsha, 1986. In view of the relationship between the atomic numberZ and the back-scattered electron generation ratio η shown in FIG. 7, inthe case where an electron beam is utilized as the exposure energy ofthe resist 3 on the substrate 1, the reflector plate 23 of the system ofFIG. 6 is preferred to be made of a plate of a heavy metal, or acommonly used ferrous metal material covered with a heavy metal, theheavy metal being selected from the group consisting of molybdenum (Mo),palladium (Pd), silver (Ag), tungsten (W), gold (Au), lead (Pb), andbismuth (Bi), so that a greater portion of the electron beam isrefelected at the reflector plate 23 and the utilization efficiency ofthe electron beam is further enhanced.

While description has been made of the particular embodiments of thisinvention, it will be understood that many modifications may be resortedto without departing from the spirit thereof. For example, although aconverging electron beam is utilized as the exposure energy beam forexposing the resist on the substrate of printed wiring boards havingthrough holes, other exposure energy beams such as ion beams and laserbeams may be utilized as the exposure energy beams. The appended claimsare contemplated to cover any such modifications as fall within the truespirit and scope of this invention.

What is claimed is:
 1. A method of exposing a resist formed on asubstrate having at least one cylindrical through hole, wherein saidresist covers a wall surface of the cylindrical through hole as well asa planar surface of the substrate, said method comprising the stepsof:irradiating an opening area of the through hole of the substrate witha converging exposure energy beam; and positioning the exposure energybeam so that the beam is incident on the opening area of the throughhole at an irradiation inclination angle with respect to a normal to theopening area of the through hole, such that a first portion of the wallsurface of the through hole is irradiated by an incident beam and asecond portion of the wall surface of the through hole is irradiated bya beam reflected from the first portion of the wall surface; whereby thefirst and second portions of the wall surface are irradiatedsimultaneously and shadow formation on the wall surface is minimized. 2.A method of exposing a resist on a substrate as claimed in claim 1,wherein said irradiation inclination angle of the beam is controlled toa predetermined angle different from 0 degrees.
 3. A method of exposinga resist on a substrate as claimed in claim 1, wherein said irradiationinclination angle of the beam is determined in accordance with a radiusand a depth of the through hole.
 4. A method of exposing resist on asubstrate as claimed in claim 1, wherein said substrate comprises asubstrate of a printed wiring board.
 5. A method of exposing a resist ona substrate as claimed in claim 1 wherein the resist is formed on thesubstrate by means of an electrodeposition method.
 6. A method ofexposing a resist on a substrate as claimed in claim 1, wherein anamount of exposure per unit area over the opening area of the throughhole is varied with respect to the amount of exposure per unit area overthe planar surface of the substrate in accordance with the depth of thethrough hole.
 7. A method of exposing a resist formed on a substrate ofa printed wiring board having at least one through hole with first andsecond openings, wherein said resist covers a wall surface of thethrough hole as well as a planar surface of the substrate, said methodcomprising the steps of:irradiating the first opening and a first wallsurface of the through hole with a converging exposure energy beamaccording to a predetermined pattern; reflecting the exposure energybeam at the second opening of the through hole by means of a reflectorplate such that a resist on the first wall surface of the through holeis exposed by the converging exposure energy beam directly incidentthereon and a resist on a second wall surface is exposed by thereflected exposure energy beam.
 8. A method of exposing a resist on asubstrate as claimed in claim 7, wherein said exposure energy beam isselected from the group consisting of an electron beam, an ion beam, anda laser beam.
 9. A method of exposing a resist on a substrate as claimedin claim 7, wherein said reflector plate is made of a heavy metalselected from the group consisting of molybdenum, palladium, silver,tungsten, gold, lead, and bismuth.
 10. A method of exposing a resist ona substrate as claimed in claim 7, wherein said reflector plate is madeof a ferrous metal covered with a heavy metal selected from the groupconsisting of molybdenum, palladium, silver, tungsten, gold, lead, andbismuth.
 11. A method of exposing a resist on a substrate as claimed inclaim 7 wherein the resist is formed on the substrate by means of anelectrodeposition method.
 12. A method of exposing a resist formed on asubstrate having at least one cylindrical through hole, wherein saidresist covers a wall surface of the cylindrical through hole as well asa planar surface of the substrate, said method comprising the stepsof:scanning the substrate at a selected speed using a convergingexposure energy beam by passing the converging exposure energy beamthrough a magnetic field; irradiating an opening area of the throughhole and the planar surface of the substrate with the convergingexposure energy beam according to a predetermined pattern so that theplanar surface of the substrate and the opening area of the through holeare exposed to selected amounts of energy; and directing the convergingexposure energy beam to the opening area of the through hole at anirradiation inclination angle with respect to a normal to the openingarea of the through hole, such that a first portion of the wall surfaceof the through hole is irradiated by the converging exposure energybeam; and reflecting the converging exposure energy beam at the firstportion of the wall surface of the through hole and directing thereflected beam to a second portion of the wall surface of the throughhole.
 13. A method of exposing a resist formed on a substrate accordingto claim 12 further comprising the step of controlling the scanningspeed of the substrate by changing the magnetic field so that the amountof exposure per unit area over the opening area of the through hole isnot less than 2h/r, where h is the height of the through hole and r isthe radius of the through hole.
 14. A method of exposing a resist formedon a substrate according to claim 12 further comprising the step ofcontrolling the intensity of the converging exposure energy beam so thatthe amount of exposure per unit area over the opening area of thethrough hole is not less than 2h/r, where h is the height of the throughhole and r is the radius of the through hole.
 15. A method of exposing aresist formed on a substrate according to claim 12 further comprisingthe step of controlling the converging exposure energy beam so that theamount of exposure per unit area of the planar substrate is differentfrom the amount of exposure per unit area over the opening area of thethrough hole.
 16. A method of exposing a resist formed on a substrateaccording to claim 12 wherein the irradiation inclination angle, δ isdefined by the following relationship: 0°≦δ≦7°.
 17. A method of exposinga resist formed on a substrate having at least one cylindrical throughhole, wherein said resist covers a wall surface of the cylindricalthrough hole as well as a planar surface of the substrate, said methodcomprising the steps of:scanning the substrate at a selected speed usinga converging exposure energy beam by passing the converging exposureenergy beam through a magnetic field; irradiating an opening area of thethrough hole and the planar surface of the substrate with the convergingexposure energy beam according to a predetermined pattern so that theplanar surface of the substrate and the opening area of the through holeare exposed to selected amounts of energy; and directing the convergingexposure energy beam to the opening area of the through hole at anirradiation inclination angle with respect to a normal to the openingarea of the through hole, such that a first portion of the wall surfaceof the through hole is irradiated by the converging exposure energybeam; reflecting the converging exposure energy beam at the firstportion of the wall surface of the through hole and directing thereflected beam to a bottom portion of the through hole which opposes theopening area of the through hole; reflecting the once reflectedconverging exposure energy beam at the bottom portion of the throughhole and directing the reflected beam to a second portion of the wallsurface of the through hole; whereby the formation of shadows on thewall surfaces of the through hole are minimized.
 18. A method ofexposing a resist formed on a substrate according to claim 17 furthercomprising the step of controlling the scanning speed of the substrateby changing the magnetic field so that the amount of exposure per unitarea over the opening area of the through hole is not less than 2 h/r,where h is the height of the through hole and r is the radius of thethrough hole.
 19. A method of exposing a resist formed on a substrateaccording to claim 17 further comprising the step of controlling theintensity of the converging exposure energy beam so that the amount ofexposure per unit area over the opening area of the through hole is notless than 2 h/r, where h is the height of the through hole and r is theradius of the through hole.
 20. A method of exposing a resist formed ona substrate according to claim 17 further comprising the step ofcontrolling the converging exposure energy beam so that the amount ofexposure per unit area of the planar substrate is different from theamount of exposure per unit area over the opening area of the throughhole.